The present invention relates broadly to a system and method for defrosting frozen objects in a microwave oven. More specifically, the present invention relates to a system and method for distinguishing between the frozen state and the thawed state of an object being heated in the cooking cavity of a microwave oven to detect the transition from the frozen state to the thawed state, and using such information to control oven operation in a defrost operating mode.
It is well known in the art that the dielectric constant for water is substantially greater than that for ice to the extent that the response of the microwave excitation system to ice in the cooking cavity generally approaches that of a no-load condition. Since generally foods contain a large percent by weight of water, the dielectric constant for a food load in its thawed state is typically substantially higher than the dielectric constant for the same load in its frozen state. In many domestic microwave cooking oven designs in present use, the ratio of input power to reflected power back to the magnetron is sensitive to variations in the dielectric constant of the food load being heated in the oven. In such ovens, it is known to monitor the microwave input reflection coefficient in the oven to detect the change in reflection coefficient indicative of the beginning of the transition of the food load from its frozen state to its thawed state. One example of such a control system and method can be found in U.S. Pat. No. 4,210,795 to Lentz. In the Lentz system the magnetron power output level is switched from a high level to a low level upon detection of a reflection coefficient less than a predetermined reference value indicating that the food load in the oven has begun to thaw.
Such an approach works satisfactorily in those microwave ovens which are particularly sensitive to changes in the dielectric constant of the food object being heated. However, the cooking performance of a microwave oven would be greatly enhanced if operating parameters of the excitation system for the oven would be relatively insensitive to variations in dielectric characteristics of food loads heated therein. An example of one such oven is described in commonly assigned U.S. Pat. No. 4,458,126 to Dills et al. In the Dills et al oven during normal operation, changes in such magnetron operating parameters as the voltage standing wave ratio and the phase of the standing wave in the waveguide for foods in the frozen and thawed states, respectively, are relatively indistinguishable when sensed by a sensor in the waveguide. Hence, an arrangement such as that described in the Lentz patent would require a very high precision measurement system capable of resolving very small changes in the measured parameters.
From the foregoing, it is apparent that the more a microwave oven system is optimized to provide a relatively stable magnetron operating point for food loads over a wide range of dielectric constant values, the more difficult it becomes to distinguish the frozen state from the thawed state for foods being defrosted in the oven based upon the difference in the operating parameters measurable in the waveguide. It would be desirable, therefore, to provide a defrost detection system for such an oven, which system effectively distinguishes between thawed and frozen states of food objects heated therein as a function of the change in the dielectric constant as the food object converts from its frozen state to its thawed state.
It is therefore a primary object of the present invention to provide a method and a system for distinguishing between the frozen state and the thawed state of a food load being heated in the cavity for a microwave oven which in normal operation demonstrates relatively little variation in voltage standing wave ratio and phase for loads over a wide range of dielectric constant values, including ice and water.